pressure builds up because of the heat going through the system, but when the system cools, it should go down. 12 hours is too long. I'd beware of a headgasket issue. It sounds like that to me - exhaust to coolant...

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (116k)
1991 350SD (206k)
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Past Diesels:
1983 300D (228K), 1985 300D (233K), 1993 300D 2.5T (338k), 1993 300SD (291k)